Known optical or optoelectronic sensors measure distances by measuring the transit time of light pulses or by comparing the phases of light beams, usually modulated light beams transmitted by the sensor to the object where they are reflected and then compared as to their phase positions with a reference light beam. The methods applied with the known measuring instruments are called "active" because part of the measuring means, usually a mirror must be mounted on the object to be measured.
In practice, therefore, the application of these known methods is limited because normally, prior to taking a measurement, the object to be measured must be approached and that can be quite troublesome in rough terrain.